In these days, in a system that boots an operating system (hereinafter OS) through a network, it has been known in general to provide a part of a physical storage device at a client terminal side, in order to reduce a load to a network boot server, with a read only cache (hereinafter read cache mechanism).
In patent document 1, a network boot system, with which a read cache mechanism is provided in the network boot system, is disclosed. In the mechanism, network accesses to a server are reduced by caching a virtual disk on a network boot server in a local disk of PC (a client terminal) that an end user uses. In such a network boot system, even when a plurality of client terminals are simultaneously booted, there are barely accesses to the network at the second boot time or later. Therefore, the booting of these client terminals becomes fast.